Optical elements being transparent for radiation provided by a laser beam with a wavelength below 250 nm, in particular with 248 nm or 193 nm, are used e.g. in microlithography projection exposure apparatuses for producing semiconductor elements. Such apparatuses generally comprise an illumination system for homogeneously illuminating a reticle mask and a projection objective for imaging a structure on the reticle mask onto a light-sensitive substrate. In a technique commonly referred to as immersion lithography, an immersion liquid is disposed between a terminating element, i.e. an optical element of the projection objective which is located adjacent to the light-sensitive substrate, and the resist (light-sensitive substrate) in order to increase the numerical aperture (NA) of the projection objective, thus allowing radiation with higher angles of incidence to reach the substrate.
The refractive index of the immersion liquid should ideally match the refractive index of the resist, being in a range of about 1.6 to 1.7. The refractive index of the optical element which is located adjacent to the light-sensitive substrate should have a comparable value and should in particular be as high as possible when an immersion fluid with a refractive index smaller than the refractive index of the resist is used. Recently, several high refractive index materials suitable for such an application have been identified.
The portion of radiation reflected by transparent optical elements, in particular high refractive index elements (nS>1.6 for a wavelength of 193 nm) should be made as small as possible over a wide range of incident angles in order to have as high a transmittance as possible. For this purpose, it is known in the art to form antireflection coatings on the surface of the substrate materials of those optical elements at least in those areas exposed to radiation.
A known type of antireflection coating for such optical elements with a high refractive index substrate consists of an arrangement having a plurality of alternating high refractive index layers and low refractive index layers.